1. Field of the Invention
The present invention relates to solar power production, and more particularly, to solar receiver panels for use in solar boilers.
2. Description of Related Art
Solar power generation has been considered a viable source to help provide for energy needs in a time of increasing consciousness of the environmental aspects of power production. Solar energy production relies mainly on the ability to collect and convert energy freely available from the sun and can be produced with very little impact on the environment. Solar power can be produced without creating radioactive waste as in nuclear power production, and without producing pollutant emissions including greenhouse gases as in fossil fuel power production. Solar power production is independent of fluctuating fuel costs and does not consume non-renewable resources.
Solar power generators generally employ fields of controlled mirrors, called heliostats, to gather and concentrate sunlight on a receiver to provide a heat source for power production. A solar receiver typically takes the form of a panel of tubes conveying a working fluid therethrough. Previous solar generators have used working fluids such as molten salt because it has the ability to store energy, allowing power generation when there is little or no solar radiation. The heated working fluids are typically conveyed to a heat exchanger where they release heat into a second working fluid such as air, water, or steam. Power is generated by driving heated air or steam through a turbine that drives an electrical generator.
More recently, it has been determined that solar production can be increased and simplified by using water/steam as the only working fluid in a receiver that is a boiler. This can eliminate the need for an inefficient heat exchanger between two different working fluids. This development has lead to new challenges in handling the intense solar heat without damage to the system. In a solar boiler, heat transfer rates can reach levels around 2-3 times the heat transfer rate of a typical fossil fuel fired boiler. This high heat transfer rate intensifies problems related to maintaining even heating and flow distribution throughout known designs of boiler panels. If flow through a portion of a receiver panel is insufficient when using water/steam as a working fluid, overheating can result for that panel portion. Such overheating can result in damage or failure of the panel and its constituent tubes if the temperatures are allowed to become severe.
Compared to typical fuel fired boilers, solar boilers have greatly varying heat inputs due to the inconsistencies in incident solar radiation. In a typical boiler the flue gas around any given tube section is nearly the same temperature, which means that the tubes absorb relatively similar amounts of energy regardless of location. Solar boilers, however, have heat input that can vary up to 50% over a relatively small area. In solar boilers made up of just a few tubes that bend around the entire structure, as in traditional fuel fired boilers, the fluid from the tubes exits at greatly different temperatures which create problematic metal temperatures and steam temperatures. This problem increases with boiler size, i.e., the bigger the boiler, the greater the temperature imbalance. This is especially true over a solar boiler panel or pass which is very wide or has a larger area, because the heat flux is highly non-uniform and can create large temperature differences in the tubes.
While the known systems of solar power production have generally been considered satisfactory for their intended purposes, there has remained a need in the art for solar receivers that can improve heat and fluid flow distribution. There also has remained a need in the art for such solar receivers that are easy to make and use. The present invention provides a solution to these problems.